Lead free solar cell contacts

ABSTRACT

Formulations and methods of making solar cells are disclosed. In general, the invention presents a solar cell contact made from a mixture wherein the mixture comprises a solids portion and an organics portion, wherein the solids portion comprises from about 85 to about 99 wt % of a metal component, and from about 1 to about 15 wt % of a lead-free glass component. Both front contacts and back contacts arc disclosed.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/145,538filed Jun. 3, 2005 which is incorporated herein in its entirety.

FIELD OF THE INVENTION

This invention relates to lead-free and cadmium-free paste compositionsand a method of making contacts for solar cells as well as other relatedcomponents used in fabricating photovoltaic cells.

BACKGROUND

Solar cells arc generally made of semiconductor materials, such assilicon (Si), which convert sunlight into useful electrical energy.Solar cells are, in general, made of thin wafers of Si in which therequired PN junction is formed by diffusing phosphorus (P) from asuitable phosphorus source into a P-type Si wafer. The side of thesilicon wafer on which sunlight is incident is generally coated with ananti-reflective coating (ARC) to prevent reflective loss of sunlight,which increases the solar cell efficiency. A two dimensional electrodegrid pattern known as a front contact makes a connection to the N-sideof silicon, and a coating of aluminum (Al) makes connection to theP-side of the silicon (back contact). Further, contacts known as silverrear contacts, made out of silver or silver-aluminum paste are printedand fired on the N-side of silicon to enable soldering of labs thatelectrically connect one cell to the next in a solar cell module. Thesecontacts are the electrical outlets from the PN junction to the outsideload.

Conventional pastes for solar cell contacts contain lead frits.Inclusion of PbO in a glass component of a solar cell paste has thedesirable effects of (a) lowering the firing temperature of pastecompositions, (b) facilitating interaction with the silicon substrateand, upon firing, helping to form low resistance contacts with silicon.For these and other reasons PbO is a significant component in manyconventional solar cell paste compositions. However, in light ofenvironmental concerns, the use of PbO (as well as CdO), in pastecompositions is now largely avoided whenever possible. Hence a needexists in the photovoltaic industry for lead-free and cadmium-free pastecompositions, which afford desirable properties using lead-free andcadmium-free glasses in solar cell contact pastes.

SUMMARY OF THE INVENTION

The present invention provides lead-free and cadmium-free glasscompositions for use in solar cell contact paste materials that providelow series resistance (Rs) and high shunt resistance (R_(sh)) to givehigh performance solar cells, as measured by efficiency (η) and fillfactor (FF). Generally, the present invention includes a solar cellcomprising a contact, made from a mixture wherein, prior to firing, themixture comprises a solids portion and an organics portion. The solidsportion comprises from about 85 to about 99 wt % of a conductive metalcomponent and from about 1 to about 15 wt % of a lead-free glasscomponent.

The compositions and methods of the present invention overcome thedrawbacks of the prior an by optimizing interaction, bonding, andcontact formation between contact components, typically silicon witheither Ag (front contact) or Al (back contact) or Ag (silver, rearcontact), through the lead-free glass medium. A conductive pastecontaining glass and silver, or glass and aluminum, is printed on asilicon substrate, and fired to fuse the glass and sinter the metaltherein. For a silver rear contact, the metal component may comprisesilver, or a combination of silver and aluminum powders and/or flakes.Upon firing, for a front contact, Ag/Si conductive islands are formedproviding conductive bridges between bulk paste and silicon wafer. In afront contact, the sequence and rates of reactions among glasses, metalsand silicon, occurring as a function of temperature are factors informing the low resistance contact between the silver paste and siliconwafer. The interface structure consists of multiple phases: substratesilicon, Ag/Si islands, Ag precipitates within the insulating glasslayer, and bulk silver. The glass forms a nearly continuous layerbetween the silicon interface and the bulk silver. For a back contact,upon firing, a p⁺ layer forms on the underlying silicon by liquid-phaseepitaxy. This occurs during the resolidification of the aluminum-silicon(Al—Si) melt. High-bismuth lead-free and cadmium-free glasses allow lowfiring temperatures in making front contacts owing to their excellentflow characteristics relatively at low temperatures. Relativelyhigh-silicon, low bismuth lead-free and cadmium-free glasses providesuitable properties for back contacts, without excessive interactionwith backside Si. Similarly, high-bismuth lead-free and cadmium-freeglasses allow the formation of suitable lead-free silver rear contactson backside Si with optimal interaction with both Si and back contact Allayer.

The foregoing and other features of the invention are hereinafter morefully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

DETAILED DESCRIPTION OF THE INVENTION

Broadly, the invention provides a solar cell contact made from a mixturewherein, prior to firing, the mixture comprises a solids portion and anorganics portion, wherein the solids portion comprises from about 85 toabout 99 wt %, preferably about 88 to about 96 wt % of a conductivemetal component, and from about 1 to about 15 wt %, preferably about 2to about 9wt % and more preferably about 3 to about 8 wt % of a glasscomponent, wherein the glass component is lead-free and cadmium-free. Asolar panel comprising any solar cell herein is also envisioned. Whenthe solar cell contact is a front contact, the metal componentpreferably comprises silver, and the glass component comprises fromabout 5 to about 85 mol % Bi₂O₃, and from about 1 to about 70 mol %SiO₂. The compositions used in making front contacts are also useful inmaking a busbar (silver rear contact) for a solar cell back contact. Asilver (or silver-aluminum) rear contact in the back makes contact withboth Si and the Al back contact layer, even though back contact Al alsodirectly contacts Si. The silver rear contact in the back contact helpsto solder connecting tabs to the solar cells that connect one cell tothe next in a solar cell module. In a back contact, the metal componentpreferably comprises aluminum, and the glass component comprises fromabout 5 to about 55 mol % Bi₂O₃, from about 20 to about 70 mol % SiO₂,and from about 0.1 to about 35 mol % B₂O₃.

Broadly, silver- and glass-containing thick film pastes are used to makefront contacts for silicon-based solar cells to collect currentgenerated by exposure to light. While the paste is generally applied byscreen-printing, methods such as extrusion, pad printing, and hot meltprinting may also be used. Solar cells with screen-printed frontcontacts are fired to relatively low temperatures (550° C. to 850° C.wafer temperature; furnace set temperatures of 650° C. to 1000° C.) toform a low resistance contact between the N-side of a phosphorus dopedsilicon wafer and a silver based paste. Methods for making solar cellsare also envisioned herein.

Aluminum- and glass-containing back contacts are used to form lowresistance ohmic contacts on the back side of the solar cell due tolarge area melting and re solidification of Al doped (p⁺) epitaxiallygrown Si layer which increases the solar cell performance due toimproved back surface field. For optimum performance a thick p⁺ re-grownregion is believed to be ideal. It is also believed that the rejectionof metallic impurities from the epitaxially growing p⁺ layer leads tohigh carrier lifetimes. These two factors are believed to increase theopen circuit voltage, and more importantly, the open circuit voltagefalls only slightly as the bulk resistivity increases. Therefore solarcell performance improves due to the formation of substantialepitaxially re grown p⁺ layer in the Al back contact. Therefore theinteraction of lead-free and cadmium-free glass in the back contactpaste, with Si should be minimal, and its interaction with Al should beenough to form a continuous Al layer without beading.

Paste Glasses. The glass component of the pastes comprises, prior tofiring, one or more glass compositions. Each glass composition comprisesoxide frits including, at a minimum. Bi₂O₃ and SiO₂. In particular, invarious embodiments of the present invention, glass compositions for afront contact may be found in Table 1. Glass compositions for backcontacts may be found in Table 2. More than one glass composition can beused, and compositions comprising amounts from different columns in thesame table are also envisioned. Regardless of the number of glasscompositions used, the total content of Bi₂O₃ and SiO₂ in the glasscomponent preferably falls within the range of about 5 to about 85 mol %Bi₂O₃ and from about 1 to about 70 mol % SiO₂. If a second glasscomposition is used, the proportions of the glass compositions can bevaried to control the extent of paste interaction with silicon, andhence the resultant solar cell properties. For example, within the glasscomponent, the first and second glass compositions may be present in aweight ratio of about 1:20 to about 20:1, and preferably about 1:3 toabout 3:1. The glass component preferably contains no lead or oxides oflead, and no cadmium or oxides of cadmium.

TABLE 1 Oxide frit ingredients for front contact glasses in molepercent. Glass Composition Ingredient I II III Bi2O3 5-85  15-80 50-80SiO₂ 1-70  2-45 15-35 ZnO 0-55 0.1-25  1-15 V₂O₅ 0-30 0.1-25  1-15

TABLE 2 Oxide frit ingredients for back contact glasses in mole percent.Glass Composition Ingredient IV V VI Bi₂O₃ 5-65  5-55 10-40 SiO₂ 15-70  20-70 30-65 B₂O₃ 0-35 0.1-35  3-20 Alkali oxides 0-35 0.1-25  5-25

In addition to the oxides of Table 1 and Table 2, additional oxides maybe included in the glass component, for example about 1 to about 20 mol% of a trivalent oxide of one or more of Al, B, La, Y, Ga, In, Ce, andCr; about 0.1 to about 15 mol % of a tetravalent oxide of one or more ofTi, Zr and Hf; about 0.1 to about 20 mol % of a pentavalent oxide of oneor more of P, Ta, Nb, and Sb, Ag₂O may be included in the silver pasteglass as a source of silver, from about 0.1 to about 12 mol %.

Metal Component. In a solar cell contact, the metal must be conductive.In a front contact, the metal component comprises silver. The source ofthe silver can be one or more fine powders of silver metal, or alloys ofsilver. A portion of the silver can be added as silver oxide (Ag₂O) oras silver salts such as silver chloride (AgCl), silver nitrate (AgNO₃)or silver acetate (AgOOCCH₃). The silver particles used in the paste maybe spherical, flaked, or provided in a colloidal suspension, andcombinations of the foregoing may be used. For example the solidsportion of the paste may comprise about 80 to about 99 wt % sphericalsilver particles or alternatively about 75 to about 90 wt % silverparticles and about 1 to about 10 wt % silver flakes. Alternatively thesolids portion may comprise about 75 to about 90 wt % silver flakes andabout 1 to about 10 wt % of colloidal silver, or about 60 to about 95 wt% of silver powder or silver flakes and about 0.1 to about 20 wt % ofcolloidal silver. Suitable commercial examples of silver particles arespherical silver powder Ag3000-1, silver flakes SF-29, and colloidalsilver suspension RDAGCOLB, all commercially available from FerroCorporation, Cleveland, Ohio.

In a back contact, the metal component comprises aluminum or alloys ofaluminum. The aluminum metal component may come in any suitable form,including those noted hereinabove for silver in the front contact.

For a silver rear contact, the metal component may comprise silver or acombination of both silver and aluminum pastes as disclosed hereinabove.

Other Additives. Up to about 30 wt % of other (i.e., inorganic)additives, preferably up to about 25 wt % and more preferably up toabout 20 wt %, may be included as needed. Phosphorus can be added to thepaste in a variety of ways to reduce the resistance of the frontcontacts. For example, certain glasses can be modified with P₂O₅ in theform of a powdered or fritted oxide, or phosphorus can be added to thepaste by way of phosphate esters or other organo-phosphorus compounds.More simply, phosphorus can be added as a coating to silver particlesprior to making a paste. In such case, prior to pasting, the silverparticles are mixed with liquid phosphorus and a solvent. For example, ablend of from about 85 to about 95 wt % silver particles, from about 5to about 15 wt % solvent and from about 0.5 to about 10 wt % liquidphosphorus is mixed and the solvent evaporated. Phosphorus coated silverparticles help ensure intimate mixing of phosphorus and silver in theinventive silver pastes.

Other additives such as fine silicon or carbon powder, or both, can beadded to control the reactivity of the metal component with silicon. Forexample these fine silicon or carbon powder can be added to the frontcontact silver paste to control the silver reduction and precipitationreaction. The silver precipitation at the Ag/Si interface or in the bulkglass, for the silver pastes in both front contacts and silver rearcontacts, can also be controlled by adjusting the firing atmosphere(e.g. firing in flowing N₂ or N₂/H₂/H₂O mixtures). Fine particles of lowmelting metal additives (i.e., elemental metallic additives as distinctfrom metal oxides) such as Pb, Bi, In, Ga, Sn, and Zn and alloys of eachwith at least one other metal can be added to provide a contact at alower temperature, or to widen the firing window. Zinc is the preferredmetal additive, and a zinc-silver alloy is most preferred for the frontcontact.

A mixture of (a) glasses or a mixture of (b) crystalline additives andglasses or a mixture of (c) one or more crystalline additives can beused to formulate a glass component in the desired compositional range.The goal is to reduce the contact resistance and improve the solar cellelectrical performance. For example, second-phase crystalline materialssuch as Bi₂O₃, Sb₂O₃, In₂O₃, Ga₂O₃, SnO, ZnO, SiO₂, ZrO₂, Al₂O₃, B₂O₃,V₂O₅, Ta₂O₅, various alumino-silicates, bismuth borates such as12Bi₂O₃.SiO₂, 2Bi₂O₃.SiO₂, 3Bi₂O₃.5SiO₂ and Bi₂O₃.4SiO₂, bismuthsilicates such as 6Bi₂O₃.SiO₂, Bi₂O₃.SiO₂, 2Bi₂O₃.3SiO₂, bismuthtitanates such as Bi₂O₃.2TiO₂, 2Bi₂O₃.3TiO₂, 2Bi₂O₃.4TiO₂, and6Bi₂O₃.TiO₂, various vanadates such as MgO.V₂O₅, SrO.V₂O₅, CaO.V₂O₅,BaO.V₂O₅, ZnO.V₂O₅, Na₂O.17V₂O₅, K₂O.4V₂O₅, 2Li₂O.5V₂O₅, and bismuthvanadates such as 6Bi₂O₃.V₂O₅, BiVO₄, 2Bi₂O₃.3V₂O₅, and BiV₃O_(q),bismuth vanadium titanates such as 6.5Bi₂O₃.2.5V₂O₅.TiO₂, zinc titanatessuch as 2ZnO.3TiO₂, zinc silicates such as ZnO.SiO₂, zirconium silicatessuch as ZrO₂.SiO₂, and reaction products thereof and combinationsthereof may be added to the glass component to adjust contactproperties. However, the total amounts of the above oxides must fallwithin the ranges specified for various embodiments disclosed elsewhereherein.

Organic Vehicle. The pastes herein include a vehicle or carrier which istypically a solution of a resin dissolved in a solvent and, frequently,a solvent solution containing both resin and a thixotropic agent. Theorganics portion of the pastes comprises (a) at least about 80 wt %organic solvent; (b) up to about 15 wt % of a thermoplastic resin; (c)up to about 4 wt % of a thixotropic agent: and (d) up to about 2 wt % ofa wetting agent. The use of more than one solvent, resin, thixotrope,and/or wetting agent is also envisioned. Although a variety of weightratios of the solids portion to the organics portion are envisioned, oneembodiment includes a weight ratio of the solids portion to the organicsportion from about 20:1 to about 1:20, preferably about 15:1 to about1:15. and more preferably about 10:1 to about 1:10.

Ethyl cellulose is a commonly used resin. However, resins such as ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose andphenolic resins, polymethacrylates of lower alcohols and the monobutylether of ethylene glycol monoacetate can also be used. Solvents havingboiling points (1 atm) from about 130° C. to about 350° C. are suitable.Widely used solvents include terpenes such as alpha- or beta-terpineolor higher boiling alcohols such as Dowanol® (diethylene glycol monoethylether), or mixtures thereof with other solvents such as butyl Carbitol®(diethylene glycol monobutyl ether); dibutyl Carbitol® (diethyleneglycol dibutyl ether), butyl Carbitol® acetate (diethylene glycolmonobutyl ether acetate), hexylene glycol, Texanol®(2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), as well as otheralcohol esters, kerosene, and dibutyl phthalate. The vehicle can containorganometallic compounds, for example those based on nickel, phosphorusor silver, to modify the contact. N-DIFFUSOL® is a stabilized liquidpreparation containing an n-type diffusant with a diffusion coefficientsimilar to that of elemental phosphorus. Various combinations of theseand other solvents can be formulated to obtain the desired viscosity andvolatility requirements for each application. Other dispersants,surfactants and rheology modifiers, which are commonly used in thickfilm paste formulations, may be included. Commercial examples of suchproducts include those sold under any of the following trademarks:Texanol® (Eastman Chemical Company. Kingsport, Tenn.): Dowanol® andCarbitol® (Dow Chemical Co., Midland, Mich.); Triton® (Union CarbideDivision of Dow Chemical Co., Midland. Mich.). Thixatrol® (ElementisCompany, Hightstown N.J.), and Diffusol® (Transene Co. Inc., Danvers,Mass.).

Among commonly used organic thixotropic agents is hydrogenated castoroil and derivatives thereof. A thixotrope is not always necessarybecause the solvent coupled with the shear thinning inherent in anysuspension may alone be suitable in this regard. Furthermore, wettingagents may be employed such as fatty acid esters, e.g.N-tallow-1,3-diaminopropane di-oleate: N-tallow trimethylene diaminediacetate; N-coco trimethylene diamine, beta diamines; N-oleyltrimethylene diamine; N-tallow trimethylene diamine; N-tallowtrimethylene diamine dioleate, and combinations thereof.

It should be kept in mind that the foregoing compositional ranges arepreferred and it is not the intention to be limited to these rangeswhere one of ordinary skill in the an would recognize that these rangesmay vary depending upon specific applications, specific components andconditions for processing and forming the end products.

Paste Preparation. The paste according to the present invention may beconveniently prepared on a three-roll mill. The amount and type ofcarrier utilized are determined mainly by the final desired formulationviscosity, fineness of grind of the paste, and the desired wet printthickness. In preparing compositions according to the present invention,the particulate inorganic solids are mixed with the vehicle anddispersed with suitable equipment, such as a three-roll mill, to form asuspension, resulting in a composition for which the viscosity will bein the range of about 100 to about 500 keps, preferably about 300 toabout 400 keps, at a shear rate of 9.6 sec⁻¹ as determined on aBrookfield viscometer HBT, spindle 14, measured at 25° C.

Printing and Firing of the Pastes. The aforementioned paste compositionsmay be used in a process to make a solar cell contact or other solarcell components. The inventive method of making solar cell front contactcomprises (1) applying a silver-containing paste to the siliconsubstrate, (2) drying the paste, and (3) firing the paste to sinter themetal and make contact to silicon. The printed pattern of the paste isfired at a suitable temperature, such as about 650-950° C. furnace settemperature, or about 550-850° C. wafer temperature. Preferably, thefurnace set temperature is about 750-930° C., and the paste is fired inair. During the firing the antireflective SiN_(X) layer is believed tobe oxidized and corroded by the glass and Ag/Si islands are formed onreaction with the Si substrate, which are epitaxially bonded to silicon.Firing conditions are chosen to produce a sufficient density of Ag/Siislands on the silicon wafer at the silicon/paste interface, leading toa low resistivity, high efficiency, high-fill factor front contact andsolar cell.

The lead-free silver pastes herein can also be used to form a backsideAg silver rear contact. A method of making a backside Ag silver rearcontact comprises: (1) applying a silver paste to the P-side of asilicon wafer in bus-bar configuration, (2) drying the paste, (3)printing and drying a Al-back contact paste, (4) applying and drying theabove mentioned silver front contact paste, and (5) co-firing all threepastes, at a suitable temperature, such as about 650-950° C. furnace settemperature; or about 550-850° C. wafer temperature.

The inventive method of making solar cell back contact comprises: (1)applying an Al-containing paste to the P-side of a silicon wafer onwhich back silver rear contact paste is already applied and dried, (2)drying the paste, and (3) applying the front contact silver paste, and(4) co-firing the front contact, silver rear contact, and Al-backcontact. The solar cell printed with silver rear contact Ag-paste.Al-back contact paste, and Ag-front contact paste is fired at a suitabletemperature, such as about 650-950° C. furnace set temperature; or about550-850° C. wafer temperature. During firing Al as the wafer temperaturerises above Al—Si eutectic temperature of 577° C., the back contact Aldissolves Si from the substrate and liquid Al—Si layer is formed. ThisAl—Si liquid continues to dissolve substrate Si into it during furtherheating to peak temperature. During the cool down period, Siprecipitates back from Al—Si melt. This precipitating Si grows as anepitaxial layer on the underlying Si substrate and forms a purer p+layer. When the cooling melt reaches Al—Si eutectic temperature theremaining liquid freezes as Al—Si eutectic layer. A purer P+ layer isbelieved to provide a back surface field (BSF), which in turn increasesthe solar cell performance. So the glass in Al-back contact shouldoptimally interact with both Al and Si without unduly affecting theformation of an efficient BSF layer.

A typical ARC is made of a silicon compound such as silicon nitride,generically SiN_(x), such as Si₃N₄, and it is generally on the frontcontact side of silicon substrate. This ARC layer acts as an insulator,which tends to increase the contact resistance. Corrosion of this ARClayer by the glass component is hence a necessary step in front contactformation. Reducing the resistance between the silicon wafer and thepaste improves solar cell efficiency and is facilitated by the formationof epitaxial silver/silicon conductive islands at the front contactAg/Si interface. That is, the silver islands on silicon assume the samecrystalline structure as is found in the silicon substrate. Until now,the processing conditions to achieve a low resistance epitaxialsilver/silicon interface have involved the use of Ag pastes that containleaded glasses. The lead free Ag-pastes and processes herein now make itpossible to produce an epitaxial silver/silicon interface leading to acontact having low resistance under broad processing conditions—a firingtemperature as low as about 650° C. and as high as about 850° C. (wafertemperature)—to produce lead free front contacts. The lead-free pastesherein can be fired in air; i.e., where no special atmosphericconditions are required.

The formation of a low resistance lead-free front contact on a siliconsolar cell is technically challenging. Both the interactions among pasteconstituents (silver metal, glass, additives, organics), and theinteractions between paste constituents and silicon substrate arecomplex, yet must be controlled. The rapid furnace processing makes allthe reactions highly dependent on kinetics. Further, the reactions ofinterest must lake place within a very narrow region (<0.5 micron) ofsilicon in order preserve the P-N junction. Similarly the formation oflead-free buck contacts on a silicon solar cell is technicallychallenging.

Method of Front Contact Production. A solar cell front contact accordingto the present invention can be produced by applying any Ag pasteproduced by mixing silver powders with lead free and cadmium-freeglasses disclosed in Table 1 to the N-side of the silicon substrate precoated with back Ag silver rear contact paste and Al back contact paste,for example by screen printing, to a desired wet thickness, e.g., fromabout 40 to 80 microns.

Method of Silver Rear Contact Production. A solar cell silver rearcontact according to the present invention can be produced by applyingany Ag paste produced by mixing silver or silver alloy powders with leadfree glasses disclosed in Table 1 to the P-side of the siliconsubstrate, for example by screen printing, to a desired wet thickness,e.g., from about 40 to 80 microns.

Method of Back Contact Production. A solar cell back contact accordingto the present invention can be produced by applying any Al pasteproduced by mixing aluminum powders with lead free glasses disclosed inTable 2 to the P-side of the silicon substrate pre coaled with silverrear contact paste, for example by screen printing, to a desired wetthickness, e.g., from about 30 to 50 microns.

Common to the production of front contacts, back contacts and silverrear contacts is the following. Automatic screen printing techniques canbe employed using a 200-325 mesh screen. The printed pattern is thendried at 200° C. or less, preferably at about 120° C. for about 5-15minutes before firing. The dry printed pattern can be co fired withsilver rear contact and Al back contact pastes for as little as 1 secondup to about 5 minutes at peak temperature, in a belt conveyor furnace inair.

Nitrogen (N₂) or another inert atmosphere may be used if desired, but itis not necessary. The firing is generally according to a temperatureprofile that will allow burnout of the organic matter at about 300° C.to about 550° C. a period of peak furnace set temperature of aboul 650°C. to about 1000° C. lasting as little as about 1 second, althoughlonger firing times as high as 1, 3, or 5 minutes are possible whenfiring at lower temperatures. For example a three-zone firing profilemay be used, with a belt speed of about 1 to about 4 meters (40-160inches) per minute. Naturally, firing arrangements having more than 3zones are envisioned by the present invention, including 4, 5, 6, or 7,zones or more, each with zone lengths of about 5 to about 20 inches andfiring temperatures of 650 to 1000° C.

Examples. Polycrystalline silicon wafers, 12.5 cm×12.5 cm, thickness250-300 μm, were coated with a silicon nitride antireflective coating onthe N-side of Si. The sheet resistivity of these wafers was about 1Ω-cm. Exemplary lead-free and cadmium-free glasses of this invention arelisted in Table 3.

TABLE 3 Exemplary Glass Compositions Glass G I J L M Mole % Bi₂O₃ 60 6075 35.8 21.57 SiO₂ 35 30 20 35.5 43.9 ZnO 5 9.7 B₂O₃ 7.2 10.0 Al₂O₃ 10V₂O₅ 5 Li₂O 10.5 Na₂O 2.5 K₂O 21.5 Nb₂O₅ 1.86

Exemplary Ag- or Al-paste formulations in Table 4 were made withcommonly used 2˜5 μm silver powders or flakes and 4˜10 μm aluminumpowders, and the organic vehicles V131, V132, V148, V205, and V450commercially available from Ferro Corporation, Cleveland, Ohio.N-Diffusol is commercially available from Transene Co. Inc., Danvers,Mass. Anti-Terra 204 is a wetting agent commercially available fromBYK-Chemie GmbH, Wesel, Germany. Cabosil® is fumed silica, commerciallyavailable from Cabot Corporation, Billerica Mass. All amounts in Table 4are in weight percent of the paste, including the solids portion and theorganics portion.

TABLE 4 Exemplary Pb-free Paste Formulations Paste 1 2 3 4 Type frontfront back silver rear contact Glass component Ingredients in wt % I J LM Glass component in paste 4.7 4.5 1.6 5 Silver 80.9 78.0 69.9 Aluminum78.2 Cabosil 0.4 Vehicle V131 1.1 3.5 10.4 Vehicle V132 8.8 13.5 14.7Vehicle V148 4.1 Vehicle V205 7.25 Vehicle V450 3.75 Texanol 7.8Anti-Terra 204 1.0 N-diffusol 0.4 0.5

The exemplary lead-free pastes in Table 4 were printed either as frontcontact or back silver rear contact or back contact on a silicon solarcell and their solar cell properties are compared to the prior art leadcontaining pastes as shown in Table 5. The other two pastes wereaccordingly commercially available front contact (CN33-462) or silverrear contact (3368, 33-451. or 33-466) or Al back contact (FX53-038, orCN53-100 or CN53-101) pastes from Ferro corporation, Cleveland, Ohio.The front contact pattern was printed using a 280 mesh screen with 100μm openings for finger lines and with about 2.8 mm spacing between thelines. The silver rear contact and back contact pastes were printedusing 200 mesh screen. The printed wafers were co-fired using a 3-zoneinfrared (IR) belt furnace with a belt speed of about 3 meters (120′)per minute, with temperature settings of 780° C. 810° C. and 930 to 970°C. for the three zones. The zones were 7′, 16′, and 7′ long,respectively. For the from contact Ag lines the fired finger width formost samples was about 120 to 170 μm and the fired thickness was about10 to 15 μm.

These lead free pastes and their comparative prior art lead pastes werefired side by side according to the aforementioned firing profile.Electrical performance of these solar cells was measured with a solartester, Model 91193-1000, Oriel Instrument Co., Stratford, Conn., underAM 1.5 sun conditions, in accordance with ASTM G-173-03. The electricalproperties of the resultant solar cells are set forth in fable 5.

TABLE 5 Properties of Solar cells made with Pb-free pastes of Table 4compared to the corresponding prior art lead containing pastes. PastePrior art Prior art Prior art Prior art 1 CN33-462 2 CN33-462 3 FX53-0384 3398 PasteType Lead free leaded Leaded Leaded Leaded Glass I J L MGlass Type Pb-free Pb Pb-free Pb Pb-free Pb Pb-free Pb Isc. A 5.6535.718 5.087 5.177 4.966 5.079 4.942 4.920 Voc, mV 601 609 610 609 600606 606 603 Efficiency, % 13.10 15.49 14.91 15.18 13.96 13.39 13.6 13.4Fill Factor, % 59.8 69.0 75.0 75.1 73.1 67.7 70.6 70.5 Rs. mΩ 21 10.08.8 8.0 11.0 14.0 14.0 13.0 Rsh. Ω 3.47 4.12 12.8 17.3 9.45 5.88 8.0 6.7

The prior art pastes 3398. CN33-462, FX53-038 are commercially availablefrom Ferro Corporation, Cleveland, Ohio. Isc means short circuitcurrent, measured at zero output voltage; Voc means open circuit voltagemeasured at zero output current; R_(S) and R_(sh) were previouslydefined. The terms Efficiency and Fill Factor are known in the art.

Table 5 clearly shows the invented lead free pastes give solar cellproperties comparable to appropriate prior art lead containing pastes.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and illustrative example shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting front the spirit or scope of the general invention concept asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A solar cell comprising a contact prepared by amixture, wherein, prior to firing, the mixture comprises: a. a solidsportion and b. an organics portion c. wherein the solids portioncomprises: i. from about 85 to about 99 wt. % of a conductive metalcomponent comprising silver, and ii. from about 1 to about 15 wt % of aglass component, wherein the glass component is lead-free and comprises(a) 50 to 80 mol % of Bi₂O₃, (b) SiO₂ and (c) V₂O₅.
 2. The solar cell ofclaim 1, wherein the glass component further comprises about 1 to about20 mol % of a trivalent oxide of an element selected from the groupconsisting of Al, B, La, Y, Ga, In, Ce, and Cr.
 3. The solar cell ofclaim 1, wherein the glass component further comprises about 0.1 toabout 15 mol % of a tetravalent oxide of an element selected from thegroup consisting of Ti, Zr and Hf.
 4. The solar cell of claim 1, whereinthe glass component further comprises about 0.1 to about 20 mol % of apentavalent oxide of an element selected from the group consisting of P,Ta, Nb. and Sb.
 5. The solar cell of claim 1, wherein the glasscomponent further comprises about 0.1 to about 25 mol % of an alkalioxide.
 6. The solar cell of claim 1, wherein the glass component furthercomprises about 0.1 to about 20 mol % of an alkaline earth oxide.
 7. Thesolar cell of claim 1, wherein the glass component further comprisesabout 0.1 to about 2.5 mol % of ZnO.
 8. The solar cell of claim 1,wherein the glass component further comprises about 0.1 to about 12 ml %Ag₂O.
 9. The solar cell of claim 1, wherein the solids portion furthercomprises a crystalline additive selected from the group consisting ofSb₂O₃, In₂O₃, Ga₂O₃, SnO, ZnO, ZrO₂, Al₂O₃, B₂O₃, Ta₂O₅, 12Bi₂O₃.SiO₂,2Bi₂O₃.SiO₂, 3Bi₂O₃.5SiO₂, Bi₂O₃.4SiO₂, 6Bi₂O₃.SiO₂, Bi₂O₃.SiO₂,2Bi₂O₃.3SiO₂, Bi₂O₃.2TiO₂, 2Bi₂O₃.3TiO₂, 2Bi₂O₃.4TiO₂, and 6Bi₂O₃.TiO₂,6Bi₂O₃.V₂O₅, BiVO₄.2Bi₂O₃.3V₂O₅, BiV₃O₉, 6.5Bi₂O₃.2.5V₂O₅.TiO₂,2ZnO.3TiO₂, ZnO.SiO₂, ZrO₂.SiO₂, MgO.V₂O₅, SrO.V₂O₅, CaO.V₂O₅, BaO.V₂O₅,ZnO.V₂O₅, Na₂O.17V₂O₅, K₂O.4V₂O₅, 2Li₂O.5V₂O₅ and reaction productsthereof and combinations thereof.
 10. The solar cell of claim 1,whereinthe solids portion further comprises about 0.5 to about 25 wt % of ametal selected from the group consisting of Bi, Zn, In, Ga, Sb andalloys thereof.
 11. The solar cell of claim 1, wherein the silvercomprises silver selected from the group consisting of flakes, powder,or colloidal particles of silver, wherein the solids portion furthercomprises phosphorus, at least a portion of which is present as acoating on at least a portion of the silver flakes, powder or colloidalparticles.
 12. The solar cell of claim 1 further comprising a silverrear contact made from a mixture wherein. prior to firing, the mixturecomprises: a. a solids portion and b. an organics portion, c. whereinthe solids portion comprises i. from about 85 to about 99 wt % of aconductive metal component, and ii. from about 1 to about 15 wt % of aglass component, wherein the glass component is lead-free.
 13. The solarcell of claim 1, wherein the glass component comprises about 75 mol %Bi₂O₃, about 20 mol % SiO₂ and about 5 mol % v₂O₅.
 14. The solar cell ofclaim 1, wherein the solar cell comprises a substrate having anantireflective coating, and wherein the contact is prepared by applyingthe mixture to the antireflective coating and firing the mixture.
 15. Asolar panel comprising the solar cell of claim 1.